A variety of microscale pressure sensing solutions have been explored in the past five decades, of which the most commonly used are piezoresistive and capacitive pressure sensors. Piezoresistive sensors typically measure stress in a diaphragm as it deflects in response to pressure, while capacitive pressure sensors respond to diaphragm deflection rather than stress. Some of the smallest micro-machined pressure sensors that have been reported—e.g., for use within cardiac catheters—use these transduction techniques. For both of these types of sensors, the side-dimensions of the diaphragms of the smallest known devices are about 1 mm. Further reduction in size has been a challenge for both approaches for a variety of reasons.
Piezoresistive sensors have relatively low output impedance, which means that the sensing circuit does not have to be located in the immediate proximity of the sensor. But reducing the diameter of the diaphragm in a piezoresistive sensor presents a challenge in localizing the resistor. For example, if the resistor extends too far from the edge toward the center of the diaphragm, there is a loss of signal due to stress averaging—i.e., the stress along the surface of the diaphragm changes from tensile stress at the perimeter to compressive stress at the center with a null point located therebetween. Making the resistor smaller is a challenge as well. Smaller resistors demand more current to generate a measurable voltage and are relatively imprecise, which affects calibration and yield. Resistors also have an inherently high temperature sensitivity, which makes this transduction approach less appealing for high temperature applications. The equivalent noise pressure from piezoresistive pressure sensors increases as 1/r4, where r is the equivalent radius of the diaphragm.
Capacitive pressure sensors present a scaling challenge because capacitance between opposing electrodes decreases in proportion to the electrode area. This scaling puts the burden of detection on the interface circuit. The interface circuit must not only be precise, but must also be located in the immediate vicinity of the sensor in order to prevent the signal—which comes from a high impedance output and is thus inherently weak—from leaking into parasitic capacitance. Another consequence of reduced capacitance is the increase in kBT/C thermal noise. Together with other noise sources, the equivalent noise pressure from capacitive pressure sensors increases as 1/r. While capacitive pressure sensors have about 1/10th the temperature sensitivity of piezoresistive devices, the proximal interface circuit must be tolerant of high temperature environments as well.